Azo dyes and liquid-crystalline materials containing these dyes

ABSTRACT

Compounds of the general formula I ##STR1## where R 1  and R 2  independently of one another are each C 6  -C 24  -alkyl or are each benzyl which is unsubstituted or substituted by alkyl or by cycloalkyl, and one of the radicals may furthermore be hydrogen, and the rings A, B, C and D can be further substituted by chlorine, methyl or methoxy, with the exception of the compound of the formula ##STR2## are useful as dyes in electrooptical liquid crystal displays or for the production of polarization films.

The present invention relates to compounds of the general formula I##STR3## where R¹ and R² independently of one another are each C₆ -C₂₄-alkyl or are each benzyl which is unsubstituted or substituted by alkylor cycloalkyl, and one of the radicals may furthermore be hydrogen, andthe rings A, B, C and D can be further substituted by chlorine, methylor methoxy, with the exception of the compound of the formula ##STR4##Alkyl radicals R¹ and R² are all conventional n-alkyl and iso-alkylradicals of 6 to 18, in particular 8 to 12, carbon atoms. Specificexamples of radicals are n-hexyl, n-heptyl, n-octyl, isooctyl, n-nonyl,isononyl, n-dodecyl, 1-ethylhexyl, 2-ethylhexyl and n-pentadecyl.

Substituted benzyl radicals can be substituted by, for example, C₁ -C₂₄-alkyl or, in particular, cyclohexyl or C₁ -C₇ -alkylcyclohexyl.

The substituted benzyl radicals are, for example, of the formula##STR5## where n is 1, 2 or 3, R³ is C₁ -C₄ -alkyl and R is C₁ -C₂₄-alkyl or is cyclohexyl which is unsubstituted or substituted by C₁ -C₇-alkyl.

Specific examples of radicals R are ethyl, n-propyl, isopropyl, n-butyl,tert.-butyl, n-pentyl, n-heptyl, n-dodecyl, 4-ethylcyclohexyl,4-n-propylcyclohexyl, 4-n-butylcyclohexyl, 4-n-pentylcyclohexyl and4-n-heptylcyclohexyl.

Compounds of the formula I can be prepared by reacting a tetrazoniumcompound of a diamine of the formula ##STR6## with identical ordifferent coupling components of the formulae ##STR7## and thenintroducing the radicals R¹ and R² by a conventional method.

The Examples which follow illustrate the preparation. Parts andpercentages are by weight, unless stated otherwise.

Particularly important compounds of the formula I are those in which R¹and R² are each benzyl which is substituted in the p-position by 4-(C₁-C₇ -alkyl)-cyclohexyl or by C₂ -C₉ -alkyl. Preferred C₁ -C₇ -alkylradicals are ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl and n-heptyl,and the corresponding isoalkyl radicals can also be used.

Other preferred radicals R¹ and R² are the isomeric C₇ -C₁₀ -radicals.Furthermore, in order to increase the solubility, mixtures of compoundsof the formula I, in which R¹ and R² can be both alkyl and substitutedbenzyl, are preferably used.

The compounds of the formula I are particularly useful as pleochroicdyes in electrooptical liquid crystal displays of the guest-host type,and for the production of colored polarization films.

Liquid-crystalline materials which contain pleochroic dyes are used indisplays. The principles of the novel use are known and are describedin, for example, H. Kelker and R. Hatz, Handbook of Liquid Crystals(1980), page 611 et seq., R. J. Cox, Mol. Cryst. Liq. Cryst. 55 (1979),51 et seq., and L. Pauls and G. Schwarz, Elektronik 14 (1982), 66 etseq. Further literature giving a detailed description of the useaccording to the invention is cited in the stated publications.

Dyes for liquid-crystalline mixtures must meet a number of requirements(cf. for example J. Constant et al., J. Phys. D: Appl. Phys. 11 (1978),479 et seq., F. Jones and T. J. Reeve, Mol. Cryst. Liq. Cryst. 60(1980), 99 et seq. and European Pat. Nos. 43,904, 55,838 and 65,869).They must not ionize in an electric field, must possess a very highmolar extinction coefficient ε and high solubility in the liquid crystalmatrix used, must be chemically and, in particular, photochemicallystable, and, as far as possible, must possess a degree of order Sgreater than 0.75 in the particular nematic phase in order for theguest-host display to exhibit good contrast.

Most of the dyes which meet these requirements belong to the classcomprising the anthraquinones (cf. for example European Pat. No. 56,492,44,893, 59,036 and 54,217).

To date, azo dyes have the disadvantage, in particular, that either thesolubility or the light-stability is unsatisfactory.

Surprisingly, the novel dyes possess a high degree of order coupled withhigh solubility and good light-stability in the particular liquidcrystal matrix.

The use of dichroic dyes for the production of polarization films isdescribed by, for example, R. Mizoguchi et al. in Displays, 4 (1983),201 et seq., where further literature is also cited.

GENERAL METHODS OF PREPARATION Example 1 ##STR8##

A mixture of 21.2 parts of 4,4'-diaminoazobenzene, 660 parts of waterand 80 parts of concentrated hydrochloric acid was filtered, 600 partsof ice were added to the filtrate, and then 60 parts of a 23% strengthaqueous sodium nitrite solution were added slowly at 5° C. When theaddition was complete, the mixture was stirred for a further 4 hours at5° C. and then added dropwise to a solution of 21.6 parts of m-cresol in500 parts of water and 8 parts of sodium hydroxide solution, whilecooling with ice. During this procedure, the pH was kept at 7 bysimultaneously adding 10% strength aqueous sodium hydroxide solution.The mixture was then stirred overnight, and the precipitate formed wasfiltered off under suction, washed with water and dried. 41 parts of4,4'-di-(2-methyl-4-hydroxyphenylazo)-azobenzene were obtained.

A mixture of 6.8 parts of this product, 2.1 parts of potassiumcarbonate, 2.6 parts of benzyl bromide and 50 parts of dimethylformamidewas stirred for 3 hours at 80° C., 100 parts of water were added at roomtemperature, and the precipitate was filtered off under suction, washedwith water and dried to give 6.9 parts of a dye mixture consisting ofthe starting compound,4-(2-methyl-4-hydroxyphenylazo)-4'-(2-methyl-4-benzyloxyphenylazo)-azobenzene(1) and 4,4'-di-(2-methyl-4-benzyloxyphenylazo)azobenzene.

The desired dye I was obtained in pure form by chromatographing the dyemixture over silica gel (MN silica gel 60, 0.063-0.2 mm) using a 10:1toluene/ethyl acetate mixture as the mobile phase, and thenrecrystallizing the product from toluene. Mp.: 191° C., λ_(max) (CH₂Cl₂): 415 nm.

Example 2 ##STR9##

A mixture of 13.5 parts of4,4'-di-(2-methyl-4-hydroxyphenylazo)-azobenzene, 8.3 parts of potassiumcarbonate, 10.8 parts of isononyl chloride and 150 parts ofdimethylformamide was stirred for 5 hours at 100° C., 300 parts of waterwere added at room temperature, and the precipitate was filtered offunder suction, washed with water and dried to give 17 parts of crudeproduct. The dialkylated product was purified as described in Example 1.Mp.: 146° C., λ_(max) (CH₂ Cl₂): 417 nm.

Other dyes prepared in a similar manner are described in the Tablebelow:

    __________________________________________________________________________                                                        λmax               Example                                         m.p.                                                                              (CH.sub.2 Cl.sub.2)       __________________________________________________________________________          ##STR10##                                 220° C.                                                                    416 nm                    4                                                                                   ##STR11##                                 244° C.                                                                    416 nm                    5                                                                                   ##STR12##                                 177° C.                                                                    417 nm                    6                                                                                   ##STR13##                                 263° C.                                                                    417 nm                    7                                                                                   ##STR14##                                 240° C.                                                                    415 nm                    8                                                                                   ##STR15##                                 198° C.                                                                    415 nm                    9                                                                                   ##STR16##                                 185° C.                                                                    417 nm                    10                                                                                  ##STR17##                                 158° C.                                                                    417 nm                    11                                                                                  ##STR18##                                 209° C.                                                                    416 nm                          ##STR19##                                                               12                                                                                  ##STR20##                                 265° C.                                                                    428 nm                    13   R.sup.1 = R.sup.2 = iso-C.sub.9 H.sub.19   158° C.                                                                    426 nm                          ##STR21##                                                               14                                                                                  ##STR22##                                 225° C.                                                                    411 nm                    15   R.sup.1  = R.sup.2 = iso-C.sub.9 H.sub.19  139° C.                                                                    410 nm                    16                                                                                  ##STR23##                                 158° C.                                                                    407 nm                    17                                                                                  ##STR24##                                 230° C.                                                                    408                       __________________________________________________________________________                                                        nm                    

Example 18 ##STR25## where R¹ and R² can be identical or different andare each iso-C₉ H₁₉ or ##STR26##

A mixture of 4.5 parts of4,4'-di-(2-methyl-4-hydroxyphenylazo)-azobenzene, 2.8 parts of potassiumcarbonate, 2.3 parts of 4-(4'-ethylcyclohexyl)-benzyl chloride and 20parts of dimethylformamide was stirred for 1 hour at 100° C., 1.8 partsof isononyl chloride were added and stirring was then continued for afurther 5 hours at 100° C. The mixture of bisalkylation products formedwas filtered off under suction when cold, washed with 20 parts ofdimethylformamide and then with water, and dried to give 5.9 parts ofcrude dye, which was purified by recrystallization from toluene. Mp.:160°-205° C., λ_(max) (CH₂ Cl₂): 416 nm.

Example 19 ##STR27## where R¹ and R² can be identical or different andare each ##STR28## which n is 2, 3, 4 or 5.

A mixture of 4.5 parts of4,4'-di-(2-methyl-4-hydroxyphenylazo)-azobenzene, 2.8 parts of potassiumcarbonate, 1.18 parts of 4-(4'-ethylcyclohexyl)-benzyl chloride, 1.25parts of 4-(4'-propylcyclohexyl)-benzyl chloride, 1.32 parts of4-(4'-n-butylcyclohexyl)-benzyl chloride, 1.39 parts of4-(4'-n-pentylcyclohexyl)-benzyl chloride and 20 parts ofdimethylformamide was stirred for 3 hours at 100° C., and the mixture ofbisalkylation products formed was filtered off under suction when cold,washed with 20 parts of dimethylformamide and then with water and driedto give 5.9 parts of crude dye, which was purified by recrystallizationfrom toluene. Mp.: 143°-162° C., λ_(max) (CH₂ Cl₂): 416 nm.

The degree of order S measured at room temperature in the broad-rangemixture ZLI 1840 from Merck for Examples 1 to 5, 8, 9, 13, 15, 18 and19, and the solubility L likewise determined in ZLI 1840 at roomtemperature, are summarized in Table 1.

Table 2 contains the solubility L and the degrees of order S measured atroom temperature in the broad-range mixture ZLI 1957/5 from Merck forExamples 10, 11, 16 and 17.

The degree of order S was determined using the conventional equation##EQU1## the measurement being carried out in commercial measuring cellswith a homogeneous edge orientation (ground PVA). The dichroic ratio CRwas determined by measuring the extinctions E" (measurement with lightpolarized parallel to the preferred direction of the nematic phase) andE.sup.⊥ (measurement with light polarized perpendicular to the preferreddirection of the nematic phase) and using the relationship

    CR=E"/E.sup.⊥

the dye concentration being chosen so that E" was from 1 to 2. Themeasurements were carried out in a Beckmann Acta CIII spectrophotometer.

FIG. 1 shows the extinctions E" and E.sup.⊥ for Example 19 in ZLI 1840.

The solubility was determined as follows:

50 mg of the particular dye were stirred in 1 ml of ZLI 1840 for 1 weekat room temperature, the saturated solution was separated off from theresidue by centrifuging, and the solubility was determined by comparisonof the extinction.

                  TABLE 1                                                         ______________________________________                                        Example     Degree of order S                                                                          Solubility L                                         ______________________________________                                        1           0.75         0.9%                                                 2           0.76         2.7%                                                 3           0.77         1.0%                                                 4           0.75         0.3%                                                 5           0.79         1.9%                                                 8           0.79         0.3%                                                 9           0.81         0.8%                                                 13          0.76           5%                                                 15          0.76           5%                                                 18          0.80         3.3%                                                 19          0.81         1.5%                                                 ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Example     Degree of Order S                                                                           Solubility L                                        ______________________________________                                        10          0.81          1.2%                                                11          0.76          0.7%                                                16          0.80            3%                                                17          0.81          0.2%                                                ______________________________________                                    

The light stability was determined by high-speed exposure of themeasuring cell in a Suntest from Hanau, at 25° C. In this procedure, thedecrease in E" was measured. The initial extinction E"_(o) and theexposure time E"_(80%) after which E"_(o) has decreased to 80% are shownin Table 3 for Examples 2, 3, 5, 9, 18 and 19. The percentage change inE"_(o) is plotted against the exposure time in FIGS. 2, 3 and 4 forExamples 2, 3 and 5. E" was determined at the stated λ_(max) values.

                  TABLE 3                                                         ______________________________________                                        Example         E.sub.o "                                                                            E.sub.80% "                                            ______________________________________                                        2               1.24   63 h                                                   3               1.95   110 h                                                  5               1.23   70 h                                                   9               1.51   140 h                                                  18              1.69   78 h                                                   19              1.61   92 h                                                   ______________________________________                                    

We claim:
 1. A trisazo dye of the formula: ##STR29## wherein R¹ and R²are different and are each independently hydrogen, C₆ -C₂₄ -alkyl or abenzyl radical having one of the formulae: ##STR30## wherein R is C₅-C₂₄ -alkyl or cycloalkyl, n is 3, R³ is C₁ -C₄ -alkyl, and wherein atleast one of R¹ or R² is the said benzyl radical; and wherein the ringsA, B, C and D, are either unsubstituted or each independentlysubstituted by chlorine, methyl, or methoxy.
 2. A trisazo dye of theformula: ##STR31## wherein R₁ and R² are each independently hydrogen, C₆-C₂₄ -alkyl, or a benzyl radical of one of the formulae: ##STR32##wherein R is C₅ -C₂₄ -alkyl or cycloalkyl, n is 3, R³ is C₁ -C₄ -alkyl,and wherein at least one of R¹ or R² is the said benzyl radical, andwherein one of the rings A, B, C or D is substituted by at least onesubstituent selected from the group consisting of chlorine, methyl ormethoxy.
 3. The compound of claim 1, wherein one of R¹ or R² isindependently benzyl substituted by C₅ -C₂₄ -alkyl or C₁ -C₇-alkylcyclohexyl, or wherein one of R¹ or R² is an isomeric C₇ H₁₅ toC₁₀ H₂₁ radical.
 4. The compound of claim 3, wherein the benzyl andcyclohexyl radicals are each substituted in the p-position.
 5. Thecompound of claim 3, wherein the said benzyl radicals are eachsubstituted by C₂ -C₇ -alkylcyclohexyl or by C₅ -C₉ -alkyl.
 6. Thecompound of claim 2, wherein ring A and ring D are substituted by atleast one methyl substituent.
 7. The compound of claim 2, wherein R¹ andR² are independently benzyl substituted by C₅ -C₂₄ -alkyl or by C₁ -C₇-alkylcyclohexyl, or wherein one of R¹ or R² is independently one of theisomeric C₇ H₁₅ to C₁₀ H₂₁ radical.
 8. The compound of claim 7, whereinthe said benzyl and cyclohexyl radicals are each substituted in thep-position.
 9. The compound of claim 7, wherein the benzyl radicals areeach substituted by C₂ -C₇ -alkylcyclohexyl or by C₅ -C₉ -alkyl.
 10. Thecompound of claim 1, wherein said C₆ -C₂₄ -alkyl substituent is a memberselected from the group consisting of n-hexyl, n-heptyl, n-octyl,isooctyl, n-nonyl, isononyl, n-dodecyl, 1-ethylhexyl, 2-ethylhexyl andn-pentadecyl.
 11. The compound of claim 1, wherein said benzyl radicalis substituted by a C₅ -C₂₄ -alkyl group, cyclohexyl or C₁ -C₇-alkylcyclohexyl.
 12. The compound of claim 1, wherein the saidcycloalkyl substituent is substituted by C₁ -C₇ -alkyl.
 13. The compoundof claim 2, wherein the said cycloalkyl substitutent is substituted byC₁ -C₇ -alkyl.